The present invention relates to a heat receiving sheet for receiving heat from heat generating bodies such as electronic components, an electronic apparatus provided with a heat receiving sheet, and a method of fabricating a heat receiving sheet.
In an electronic apparatus such as a desktop computer, a notebook computer, and a mobile communication apparatus, electronic components such as a CPU element, coil elements, and capacitors are mounted on a printed circuit board. In recent years, with progresses in the processing speed, the functionality, and the performance of electronic apparatuses, the heat generation during the operation of these electronic components tends to increase. Thus, in order that stable operation should be achieved in the electronic apparatuses, the heat generated by the electronic components need be released rapidly to the outside. That is, heat releasing performance need be improved.
In the case that a sufficient space is available over the electronic components, techniques of directly cooling the electronic components are adopted. These techniques include: an air cooling method using a heat sink and a cooling fan; and a liquid cooling method using a passage through which a coolant is circulated. In contrast, in the case that a space for such cooling members is not available over the electronic components, a metal or carbon heat transfer plate or alternatively a heat pipe is provided so that the heat is transported from the electronic components to a location where a heat sink or a cooling fan is installed.
However, when a plurality of electronic components having diverse heights are mounted on the same substrate, fabrication and installation are difficult for such a heat transfer plate or a heat pipe. However, in such a case, a method is known that the entire substrate is covered with a several-mm-thick heat receiving sheet composed of silicone rubber having a relatively high thermal conductivity, so that the heat from the electronic components is diffused (see, for example, Japanese Patent Application Laid-Open No. 11-26968 (1999)).
This method employing a heat receiving sheet composed of silicone rubber has an advantage that a larger number of electronic components can be cooled using a small space in comparison with the method of employing a metal heat sink on an electronic component that generates heat. Nevertheless, the following problems arise at the same time. When the heat receiving sheet is heated, and when its temperature goes high, a thermally weak electronic component can conversely receive the heat from the heat receiving sheet, so that the property of the electronic component can degrade. Further, the electronic components mounted on the printed circuit board have diverse heights depending on the kind of the component. Thus, a high pressure acts on a large-height electronic component, and thereby causes damage to the bonding portion between the electronic component and the printed circuit board. In order to avoid the damage caused by the high pressure, a heat receiving sheet having flexibility is useful. Nevertheless, when the orientation of the electronic apparatus is changed, the flexibility can cause position slippage or removal of the heat receiving sheet. On the other hand, in a small-height electronic component, the pressure is insufficient, and so is the contact with the heat receiving sheet. This causes the problem of a low cooling efficiency.